The present invention relates to technology of a fail-safe control of a sliding mode control system, for example, to technology for performing a fail-safe control at an abnormal time of a sliding mode control system used for feedback controlling a rotation phase of a camshaft relative to a crankshaft to a target value.
A conventional valve timing apparatus is known as a vane type valve timing controlling apparatus disclosed in Japanese Unexamined Patent Publication 10-141022 such as an apparatus for successively changing opening and closing timing of intake and exhaust valves by changing rotation phase of a camshaft relative to a crankshaft.
This apparatus forms concave portions in the inner surface of a cylindrical housing fixed to a cam sprocket in which vanes of an impeller are accommodated in the concave portions so that the camshaft can rotate relative to the cam sprocket within the range where the vanes of the impeller can move in the concave portions.
By relatively supplying and discharging oil into a pair of oil pressure chambers formed by defining the concave portions in the front and the rear of the rotation direction, the vanes are held in the mid position of the concave portions and thus successive changing of rotation phase can be carried out. Thus, supply and discharge of the oil is feedback controlled by PID control so that a rotation phase of a control object corresponds to a target value.
When the rotation phase cannot be controlled to the target value in the above valve timing control apparatus, the operation performance of an engine is deteriorated. Therefore, in the conventional valve timing apparatus, a fail-safe control has been performed for diagnosing the existence of abnormality and for controlling the rotation phase of the camshaft relative to the crankshaft to the most retarded side when the occurrence of abnormality is diagnosed.
However, the above fail-safe control for controlling the rotation phase to the most retarded side is of an inappropriate constitution, since only a minimum operation performance can be achieved.
Further, it is preferable to variably set a feedback gain in accordance with an oil temperature and an oil pressure in order to execute the PID control with a good response characteristic. However, gain matching is not easily performed. Accordingly, the shift from the general PID control to a sliding mode control with small disturbance is being investigated.
In view of the foregoing, the present invention has been achieved and has an object of carrying out a fail-safe control capable of maintaining high performance as possible on abnormality of a control system.
Another object of the present invention is to carry out a fail-safe control corresponding to the types and degree of abnormality of a control system.
In order to achieve the above objects, the present invention comprises the following constitution in a system for feedback controlling a control object to a target value by calculating a control amount composed of a linear term and a non-linear term by a sliding mode control.
It is judged whether or not an abnormality occurs in the system.
When it is judged that the system is abnormal, a feedback control is carried out using only a non-linear term calculated by the sliding mode control.
According to this constitution, by using only the non-linear term, the feedback control is carried out at a slower speed than in a normal time. As a result, a filter function is generated to instantaneous breakdown, noises, and fluctuations of control signal, so that the control object can be converged to the target value to achieve a desired performance restraining abrupt fluctuations, while restraining abrupt fluctuations.
Also, the constitution may be such that the feedback control may be carried out using the non-linear term only when it is judged that the sliding mode control system is in a predetermined abnormal state.
According to this constitution, at an abnormal state where the feedback control cannot be carried out due to a steady failure such as a break down of control signal, the fail-safe control is carried out by a feedforward control. Only when the feedback control can be carried out, the feedback control is carried out using only the non-linear term. Thereby, the feedback control can be carried out according to a degree of abnormality.
Further, the constitution may be such that the predetermined abnormality includes an abnormality which occurs transiently.
According to this constitution, the feedback control can be carried out using only the non-linear term to the abnormality which occurs transiently, such as instantaneous breakdown and noises of control signal.
Accordingly, the control object can be feedback controlled to the target value while preventing abrupt fluctuations or operating errors caused by the abnormality occurring transiently.
Also, the predetermined abnormality may include an abnormality where a deviation between the target value and the actual value of the control object exceeds a threshold value for a predetermined time or above.
According to this constitution, when a good response is not obtained, the feedback control using only the non-linear term can be carried out to secure the good performance while preventing abrupt fluctuations.
The control object may be a rotation phase of a camshaft relative to a crankshaft of an internal combustion engine wherein the opening and closing timing of an intake valve and an exhaust valve is controlled variably and continuously by feedback controlling the rotation phase to a target value.
In this way, in a constitution wherein the valve timing is continuously changed by changing the rotation phase of the camshaft relative to the crankshaft, the valve timing (substantial control object) is feedback controlled to a target value by a sliding mode control and also the presence of abnormality is judged to carry out the feedback control by using only the non-linear term at the time of occurrence of abnormality.
Only when it is judged that there is a slight abnormality in a sliding mode control system in which the control object is the rotation phase of the camshaft relative to the crankshaft of the internal combustion engine, the feedback control using only the non-linear term is carried out. When it is judged that the sliding mode control system has a crucial abnormality, the rotation phase of the camshaft may be controlled fixedly to the most retarded side.
In this way, when the system has a slight abnormality such as an instantaneous breakdown, noises and a bad response of signal necessary for detection of the rotation phase of the camshaft, the feedback control using only the non-linear term is carried out. When the system has an abnormality wherein it is impossible to carry out the feedback control due to a steady failure such as a breakdown of signal, the rotation phase of the camshaft can be controlled fixedly to the most retarded side to secure the required minimum performance.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.